Robust heterodyne interferometer optical gauge

ABSTRACT

A method for performing optical signal and beam distribution in a heterodyne interferometer. A planar lightwave circuit is provided including a plurality of waveguide optical transmission elements and an input coupler and an output coupler arranged along the optical transmission elements. Optical pathlengths of the transmission elements are matched between the input coupler and the output coupler to compensate for thermal effects. Reference and measurement optical phases are determined employing the input coupler and the output coupler.

FIELD OF THE INVENTION

[0001] The present invention relates to a robust heterodyneinterferometer for very high resolution non-contact measurement ofdistances.

BACKGROUND OF THE INVENTION

[0002] Heterodyne Interferometry measures the phase of a continuoussignal for as long as the signal remains stable over all portions of areference path and a measurement path. The major practical problem withhigh resolution interferometry is that every optical pathlength changein the system, including the ones that are not intended, are measured.After the beams are split in the interferometer, movement of opticalcomponents, especially mirrors, add or subtract optical path length fromone of the two beams separately, resulting in a signal that is unrelatedto the measurement. Therefore, stability of optical components in theseparate legs of the interferometer is critical, as described in J. D.Trolinger, Ultra High Resolution Interferometry, Proc. SPIE Vol. 2816,pp114-123 (1996).

[0003] Typically, existing heterodyne interferometer devices are subjectto thermal drift errors that limit their performance. In addition, theyconsist of a precision assembly of critical components leading to highcost to manufacture. Therefore, a need exists for a robust, easilymanufacturable device, which is immune to thermal drift errors.

SUMMARY OF THE INVENTION

[0004] The present invention replaces many of the optical components,their mountings and mechanically variable paths in a typical heterodyneinterferometer with photolithographically defined components in aninherently stable single mode planar optical waveguide circuit. Thewaveguide circuit is a planar lightwave circuit (PLC) fabricated insilica on silicon. Such circuits are described in M. Kawachi, Silicawaveguides on silicon and their application to integrated-opticcomponents, Optical and Quantum Electronics, Vol. 22, pp391-416 (1990).Other material systems are possible but these have been chosen for theirmechanical robustness, thermal stability, low coefficient of thermalexpansion and commercial fabrication process maturity.

[0005] The present invention concerns a PLC containing at least twocirculating optical waveguide circuits. The two circuits arenon-interfering until combined at an output coupler. The circuitcontains various waveguide couplers and splitters to provide measurementand reference signals to the output couplers. Circuit or waveguide pathson the PLC are photolithographically defined so that thermally inducedpathlength differences between the two non-interfering circuits areprecisely compensated. The PLC is incorporated into a system thatcontains a laser source or sources, a means of deriving two preciselyseparated optical frequencies from the laser source or sources, fiberoptic connection of the two optical frequency signals to the PLC, meansfor coupling the measurement optical signal off of and back into the PLCand fiber coupling of the interfering optical signals output to opticalintensity detectors.

[0006] Still other objects and advantages of the present invention willbecome readily apparent by those skilled in the art from a review of thefollowing detailed description. The detailed description show anddescribes preferred embodiments of the present invention, simply by wayof illustration of the best mode contemplated of carrying out theinvention. As will be realized, the present invention is capable ofother and different embodiments and its several details are capable ofmodifications in various obvious respects, without departing from theinvention. Accordingly, the drawings and description are illustrative innature and not restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] Objects and advantages of the present invention will be moreclearly understood from the following specification when considered inconjunction with the accompanying drawings, in which:

[0008]FIG. 1 represents an embodiment of a heterodyne interferometeraccording to the present invention wherein separate measurement andreference signals are obtained;

[0009]FIG. 2 represents a schematic drawing of an embodiment of a planarlightwave circuit included in the embodiment of a heterodyneinterferometer represented in FIG. 1;

[0010]FIG. 3 represents another embodiment of a heterodyneinterferometer according to the present invention wherein twomeasurement signals contain oppositely signed reference signals;

[0011]FIG. 4 represents a schematic drawing of an embodiment of a planarlightwave circuit included in the embodiment of a heterodyneinterferometer represented in FIG. 3;

[0012]FIG. 5 represents a schematic drawing of an embodiment of a planarlightwave circuit including two measurement beams and an additionalreference signal output;

[0013]FIG. 6 represents a schematic drawing of another embodiment of aplanar lightwave circuit including two measurement beams and anadditional reference signal output as in the embodiment shown in FIG. 5but with the reference output on the opposite side of the planarlightwave circuit from the embodiment shown in FIG. 5;

[0014]FIG. 7 represents a perspective partially cut-away view of anembodiment of a planar lightwave circuit according to the presentinvention as represented in FIGS. 3 and 4;

[0015]FIG. 7a represents a cross-sectional view of a section of theembodiment of the planar lightwave circuit shown in FIG. 7;

[0016]FIG. 8 represents a schematic drawing of an embodiment of abalanced detector common to the some embodiments of balanced outputs ofvariations of planar lightwave circuits and corresponding versions ofheterodyne interferometers as represented in FIGS. 1-6;

[0017]FIG. 9 represents a graph that illustrates calculationsdemonstrating the effectiveness of reflected signal reduction at angledinterfaces;

[0018]FIG. 9a represents a cross-sectional view of a portion of a planarlightwave circuit on which the calculations shown in FIG. 9 were made;

[0019]FIG. 10 represents a perspective partially cut-away view of anembodiment of a TM mode stripper incorporated into a planar lightwavecircuit according to the present invention as represented in FIGS. 1-6;

[0020]FIG. 10a represents a cross-sectional view of the embodiment of aTM stripper shown in FIG. 10 along the line A-A;

[0021]FIG. 11 represents a graph that illustrates calculationsillustrating the effectiveness of the TM mode stripper as a function ofoptical waveguide to metal film separation; and

[0022]FIG. 12 represents a cross-sectional view of a portion of a planarlightwave circuit on which the calculations shown in FIG. 11 were taken.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention provides a solution to problems existingwith known devices. Along these lines, the present invention can providea small, lightweight and robust optical gauge. A planar lightwavecircuit (PLC) according to the present invention replaces large bulkcomponents typically utilized in known devices. Use of the PLC accordingto the present invention results in smaller devices that are lesssensitive or insensitive to thermal changes. Additionally, a deviceaccording to the present invention can be fiber coupled to inputs andoutputs, such as sensors and light sources, and contains few components.Features of the present invention can permit devices according to thepresent invention to measure distance changes, for example, that are onthe order of about one-millionth of the wavelength of light utilized inan application of the device.

[0024] The present invention can provide a device that can be utilizedin the control of large structures by providing position, velocity, andacceleration data for control loops, characterization of turbulent flowby measuring velocity spectra, characterization of sound by measurementof motion of surfaces, among other variables. Due to its size,construction, and operating characteristic, the present invention isparticularly suited to use for control of large, remotely controlledoptical systems. Devices according to the present invention could alsobe utilized on assembly lines, where process controls require thenon-contact control of positions and velocity. Devices according to thepresent invention are particularly useful where a rugged design isrequired.

[0025]FIG. 1 illustrates an embodiment of a Planar Lightwave Circuit(PLC) according to the present invention. This type of circuit is alsocommonly referred to as a Photonic Integrated Circuit (PIC), a LightwaveIntegrated Optic (LIO) circuit or an Integrated Optic (IO) circuit amongother common nomenclatures in the literature. PLC fabrication technologyhas been developed in a number of material systems including glasses,polymers, lithium niobates and III-V semiconductors. Among the glassesare high silica (SiO₂) glasses with or without additional network formeroxides such as GeO₂, TiO₂, P₂O₅ and/or B₂O₃. These glasses and thecircuit components therein may be deposited onto various substrates suchas silicon (Si) or quartz. (SiO₂).

[0026] The present invention will be described in terms of the silica onsilicon technology due to the mature state of commercial development ofthis material system along with the desirable mechanical, optical andthermal properties of this material system. However, the presentinvention can be realized in any of the material systems available forPLC development. The silica on silicon PLC may be fabricated by any ofthe processes available for this material system including but notlimited to flame hydrolysis deposition (FHD), vacuum deposition (VD) andvarious chemical vapor deposition processes (CVD) including low pressureCVD (LPCVD), atmospheric pressure CVD (APCVD) and plasma enhanced CVD(PECVD).

[0027] A PLC according to the present invention can include single modeoptical waveguides with waveguide bends, waveguide crossings, waveguidecouplers and waveguide splitters. The waveguide bends typically areconstrained to radii of curvature sufficiently large to yield acceptablylow radiation loss from the bends. This radiation loss will bedetermined by the refractive index difference between the waveguide coreand cladding materials (the “index contrast”) and will be fixed by theparticular waveguide properties chosen. Waveguide crossing angles may bedetermined by a minimum crossing angle that provides an acceptably lowlevel of crosstalk between the two crossing waveguides, which is afunction of the index contrast. Minimum bend radii and minimum crossingangles may determine the minimum physical size of the PLC.

[0028] Waveguide couplers and/or splitters are identified in theembodiments of FIGS. 1-6 by the letters A, B, E, F, G, H and I.Splitters A, and B in FIGS. 1 and 2 along with splitters G and H inFIGS. 5 and 6 may be Y-branch splitters, directional couplers ormultimode interference (MMI) devices. All output couplers, E, H and I,may be directional couplers or MMI devices to provide the indicatedbalanced output. Directional couplers only will be described in thevarious embodiments of the present invention although Y-branch or MMIdevices represent examples of applicable alternatives.

[0029]FIG. 1 schematically represents one embodiment of a heterodyneinterferometer incorporating a PLC. The PLC is within the rectangularregion outlined by the broken line. The distance to be measured is theseparation between mirrors MR and MB. The measurement beam may beconsidered to enter the PLC at coupler B, traverse the path BC on thePLC, exit the PLC at C, pass though a collimating lens and reflect frommirror MR onto mirror MB. From mirror MB, the beam is reflected backonto the PLC at D after passing though another collimating lens. Thebeam then traverses path DE on the PLC and exits the PLC at coupler E.

[0030] The path length on the PLC of the measurement beam is BC+DE andthe external path length of the measurement beam ISC_(MR)+M_(R)M_(B)+M_(B)D. A reference path for the measurement beam isestablished by the path length BI. The phase of the measurement opticalsignal entering the PLC at coupler at B is uncontrolled and variable dueto phase noise in the laser source, mechanical and thermal effects inthe fiber coupling the laser to the PLC and coupling effects between thefiber and laser or PLC. This unknown and uncontrolled optical phase atthe input to coupler B may be defined as φ_(R)(t).

[0031] The heterodyne optical signal may be considered to enter the PLCat coupler A. A portion of the beam exits the PLC through themeasurement coupler output at E after traversing the pathlength AE onthe PLC. In this embodiment, the path length AE is selected such thatAE=BC+DE so that thermal variations of measurement signal path on chipare equal to thermal variations of heterodyne signal path. A referencepath for the heterodyne optical signal may be established by the pathlength AI such that AI=BI so that thermal variations of these twooptical paths are also matched.

[0032] The phase of the heterodyne optical signal entering the PLC atcoupler at A may be uncontrolled and variable due to phase noise in thelaser source, phase noise in the offset frequency generator, mechanicaland thermal effects in the fiber coupling the laser to the acousto-opticfrequency shifter, fiber coupling the acousto-optic frequency shifter tothe PLC and coupling effects between the fibers and laser, acousto-opticfrequency shifter or PLC. This unknown and uncontrolled optical phase atthe input to coupler A may be defined as φ_(B)(t). The embodiment of aninterferometer shown FIG. 1 and its corresponding PLC schematic shown inFIG. 2 will be referred to herein as a “Racetrack” configuration sincethe measurement beam exits the PLC at port C, traverses a loop aroundthe PLC and reenters the PLC through port D. This configuration easilyaccommodates the required PLC pathlength equalities, AE=BC+DE and AI=BI,by appropriate location of the couplers A and B on the PLC along with anextra “bulge” in the path AE to lengthen the inside track of AE.

[0033] In the embodiment of FIGS. 1 and 2, the optical intensityinterference pattern at the reference signal output coupler, I, is givenby the following formula 1:

P _(ref±)=(P _(R) +B± 2{square root}{square root over (P _(R) P _(B))}cos[(φ _(R)−φ_(B))t+(φ_(R)φ_(B))+(θ_(BI)−θ_(AI))])/4  (1)

[0034] where P_(R) is the optical power, OR is the optical frequency andφ_(R)(t) is the optical phase at the input to coupler B while P_(B) isthe optical power, ω_(B) is the optical frequency and φ_(B)(t) is theoptical phase at the input to coupler A. Formula 1 does not assume thatany optical loss will occur. Optical loss would only appear as areduction in the values of P_(R) and P_(B). It would not affect thephase terms that are of interest in the heterodyne interferometer. Theupper (minus) sign in the equation corresponds to the optical power inthe upper waveguide exiting the coupler I while the lower (plus) signcorresponds to the optical power in the lower waveguide exiting thecoupler I.

[0035] The phase terms, θ_(BI) and θ_(AI) correspond to the optical pathlengths θ_(BI)=ω_(R)n_(eff)BI/c and θ_(AI)=ω_(B)n_(eff)AI/c wheren_(eff) is the effective refractive index of the guided optical wavesand c is the velocity of light in vacuum. The difference of these twophase terms in the output heterodyne interference reference signal maybe defined by the following formula 2:

θ_(BI)−θ_(AI) =n _(eff)(ω_(R) BI−ω _(B) AI)/c=ω _(R) n_(eff)(BI−AI(1+Δω)/ω_(R)))/c  (2)

[0036] where Δω is the offset frequency difference, ω_(B)−ω_(R), of thesignal driving the acousto-optic frequency shifter. Since ω_(R)≈1.2×10¹⁵radians per second and Δω10⁵ to 10⁸ radians per second, the terminvolving Δω/ω_(R)≈10⁻⁷ to 10⁻¹⁰ may be neglected. As a result,θ_(BI)−θ_(AI)≈ω_(R)n_(eff)(BI−AI)/c≡θ_(I) where θ₁ is a small,temperature insensitive phase angle since BI≈AI by design.

[0037] A similar equation for the optical intensity interference patternat the measurement signal output coupler, E, may be given by thefollowing formula 3

P _(meas±)=(P _(R) +P _(B) ±2{square root}{square root over (P _(R) P_(B) )}cos[(ω _(R)−ω_(B))t+(φ_(R)−φ_(B))+θ_(E)+2φ_(L)])/4  (3)

[0038] where θ_(E)≡θ_(BC+DE)−θ_(AE) is a small, temperature insensitivephase angle since AE≈BC+DE by design and2φ_(L)=ω_(R)(CM_(R)+M_(R)M_(B)+M_(B)D)/c contains the external pathlength of the measurement beam. Comparing the phases of the measurementoptical signal interference pattern, P_(meas±), and reference signalinterference pattern, P_(ref±), we see that the unknown, uncontrolledphase function, φ_(R)−φ_(B) may be determined from P_(ref±) andsubtracted from the phase of P_(meas±) to obtain 2φ_(L).

[0039]FIG. 3 schematically represents another embodiment according tothe present invention of a heterodyne interferometer incorporating aPLC. The PLC is contained within the rectangular region defined by thebroken line. FIG. 4 schematically represents the embodiment of the PLCcircuit incorporated in the interferometer of FIG. 3. The distance to bemeasured is the sum of the distance from the PLC at C to the mirror atL_(R) and the distance from the PLC at D to the mirror at L_(B). Oneoptical source may be considered to enter the PLC at coupler B withoptical power, P_(R), optical frequency, φ_(R), and optical phase,φ_(R)(t). In this embodiment, the phase of the measurement opticalsignal entering the PLC at coupler B, φ_(R)(t), is uncontrolled andvariable due to phase noise in the laser source, mechanical and thermaleffects in the fiber coupling the laser to the PLC and coupling effectsbetween the fiber and laser or PLC. A second optical source may beconsidered to enter the PLC at coupler A with optical power, P_(B),optical frequency, φ_(B), and optical phase, φ_(B)(t). In thisembodiment, the phase of the measurement optical signal entering the PLCat coupler A, φ_(B)(t), is uncontrolled and variable due to phase noisein the laser source, phase noise in the offset frequency generator,mechanical and thermal effects in the fiber coupling the laser to theacousto-optic frequency shifter, the fiber coupling the acousto-opticfrequency shifter to the PLC and coupling effects between the fibers andlaser, acousto-optic frequency shifter or PLC.

[0040] One branch of the path of the measurement optical signal enteringthe PLC at B traverses the path BC, exits the PLC at C and after anexternal path of 2L_(R), reenters the PLC at C and traverses the pathCBF to the output coupler at F. The pathlength of this signal on the PLCis 2BC+BF. One branch of the path of the measurement optical signalentering the PLC at A traverses the path AF to the output coupler at F.These two pathlengths are set equal, i.e. 2BC+BF=AF to compensatethermal effects on the PLC.

[0041] The other branch of the measurement optical signal entering thePLC at B traverses the path BE to the output coupler at E. The otherbranch of the measurement optical signal entering the PLC at A traversesthe path AD, exits the PLC at D and after an external path of 2LB,reenters the PLC at D and traverses the path DAE to the output couplerat E. The pathlength of this signal on the PLC is 2AD+AE. These latertwo pathlengths are also set equal, i.e. 2AD+AE=BE to compensate thermaleffects on the PLC. The interferometer of FIG. 3 and its correspondingPLC schematic shown in FIG. 4 are referred to herein as the “Trombone”since the required PLC pathlength equalities, 2BC+BF=AF and 2AD+AE=BEare easily accommodated by appropriate adjustment of the lengths of thehorizontal straight segments in FIG. 4.

[0042] In the embodiment shown in FIGS. 3 and 4, the optical intensityinterference pattern at the optical signal output coupler, E, may bedefined by the following formula 4 $\begin{matrix}{P_{E \pm} = {\frac{P_{R}}{4} + {\frac{P_{B}}{8} \pm {\frac{\sqrt{P_{R}P_{B}}}{2^{3/2}}{\sin \left\lbrack {{\left( {\omega_{R} - \omega_{B}} \right)\quad t} + \quad \left( {\varphi_{R} - \varphi_{B}} \right) + \left( {\theta_{BE} - {2\theta_{AD}} - \theta_{AE}} \right) - {2\varphi_{{DL}_{R}}}} \right\rbrack}}}}} & (4)\end{matrix}$

[0043] where P_(R) is the optical power, ω_(R) is the optical frequencyand φ_(R)(t) is the optical phase at the input to coupler B while P_(B)is the optical power, ω_(B) is the optical frequency and φ_(B)(t) is theoptical phase at the input to coupler A as indicated in FIG. 4. Nooptical loss has been assumed in this expression. Optical loss wouldonly appear as a reduction in the values of P_(R) and P_(B). It wouldnot affect the phase terms that are of interest in the heterodyneinterferometer. The upper (plus) sign in the equation corresponds to theoptical power in the upper waveguide exiting the coupler E while thelower (minus) sign corresponds to the optical power in the lowerwaveguide exiting the coupler E.

[0044] The phase terms, θ_(BE) and 2θ_(AD)+θ_(AE) correspond to theoptical path lengths θ_(BE)ω_(R)n_(eff)BE/c and2θ_(AD)+θ_(AE)=ω_(B)n_(eff)(2AD+AE)/c, where n_(eff) is the effectiverefractive index of the guided optical waves and c is the velocity oflight in vacuum. The difference of these two phase terms in the outputheterodyne interference reference signal may be defined by the followingformula 5

θ_(BE)−θ_(AD)−θ_(AE)=n_(eff)(ω_(R)BE−ω_(B)(2AD+AE))/c=ω_(R)n_(eff)(BE−(2AD+AE)(1+Δω/ω_(R)))/c  (5)

[0045] where Δω is the offset frequency difference, ω_(B)−θ_(R), of thesignal driving the acousto-optic frequency shifter. Since ω_(R)≈1.2×10¹⁵radians per second and Δω≈10⁵ to 10⁸ radians per second, the terminvolving Δω/ω_(R)≈10⁻⁷ to 10⁻¹⁰ maybe neglected. Also,θ_(BE)−θ_(AD)−θ_(AE)≈θ_(R)n_(eff)(BE−(2AD+AE))/c≡θ_(E), where θ_(E) is asmall, temperature insensitive phase angle since BE≈2AD+AE by design.The phase term 2φ_(DL) _(B) =2ω_(B)L_(B)/c contains the round trippathlength measurement from the PLC at exit point D to the lower mirrorin FIG. 2 and back to point D.

[0046] A similar equation for the optical intensity interference patternat the optical signal output coupler, F, is given by the followingformula 6 $\begin{matrix}{P_{F \pm} = {\frac{P_{R}}{4} + {\frac{P_{B}}{8} \pm {\frac{\sqrt{P_{R}P_{B}}}{2^{3/2}}{\sin \left\lbrack {{\left( {\omega_{R} - \omega_{B}} \right)\quad t} + \quad \left( {\varphi_{R} - \varphi_{B}} \right) + \left( {{2\theta_{BC}} + \theta_{BF} - \theta_{AF}} \right) + {2\varphi_{{CL}_{R}}}} \right\rbrack}}}}} & (6)\end{matrix}$

[0047] where θ_(F)≡2θ_(BC)+θ_(BF)−θ_(AF) is a small, temperatureinsensitive phase angle since AF≈2BC+BF by design. The upper (plus) signin the equation corresponds to the optical power in the upper waveguideexiting the coupler F while the lower (minus) sign corresponds to theoptical power in the lower waveguide exiting the coupler F. The phaseterm 2φ_(DL) _(R) =2ω_(R)L_(R)/c contains the round trip pathlengthmeasurement from the PLC at exit point C to the upper mirror in FIG. 2and back to point C. If the phase of the heterodyne signal out ofcoupler E is subtracted from the phase of the heterodyne signal out ofcoupler F, the result 2(φ_(CL) _(R) +φ_(DL) _(B) )+θ_(F)−θ_(E) isobtained, which contains the separation of the two mirrors plus twosmall, fixed, temperature insensitive terms, θ_(F) and θ_(E).

[0048] While the interferometer configuration shown in FIG. 3 is suchthat measurement of the sum of the two distances, L_(R) and L_(B) allowselimination of the uncontrolled phase fluctuations at the inputcouplers, φ_(R)−φ_(B), it is also possible to add a separate measurementcircuit to determine this quantity as shown the schematicrepresentations of PLC circuits shown in FIGS. 5 and 6.

[0049] In FIG. 5, the additional couplers G and H are arranged such thatthe input phase signal is coupled out on the left side of the PLC. Thisembodiment is referred to herein as the “Lightbulb”. The design andoperation of this circuit is essentially the same as the “Trombone”except that some optical power is coupled out at I to provide thereference signal. If the optical signal inputs are as described for theTrombone, P_(R)(ω_(R), φ_(R)) into coupler B and P_(B)(φ_(B), φ_(B))into coupler A, then the optical intensity interference pattern at theoptical signal output coupler, E, is given by the following formula 7$\begin{matrix}{P_{E \pm} = {\frac{P_{R}}{8} + {\frac{P_{B}}{8} \pm {\frac{\sqrt{P_{R}P_{B}}}{4}{{\sin \left\lbrack {{\left( {\omega_{R} - \omega_{B}} \right)\quad t} + \quad \left( {\varphi_{R} - \varphi_{B}} \right) + \left( {\theta_{{BG} + {GE}} - {2\theta_{AD}} - \theta_{AE}} \right) - {2\varphi_{{DL}_{R}}}} \right\rbrack}.}}}}} & (7)\end{matrix}$

[0050] The upper (plus) sign in the equation corresponds to the opticalpower in the upper waveguide exiting the coupler E while the lower(minus) sign corresponds to the optical power in the lower waveguideexiting the coupler E. The phase angle θ_(BG+GE)−2θ_(AD)−θ_(AE)≡θ_(E) ismade small and temperature invariant or insensitive by setting thepathlengths BG+GE=2AD+AE.

[0051] The optical intensity interference pattern at the optical signaloutput coupler, F, is given by the following formula 8 $\begin{matrix}{P_{F \pm} = {\frac{P_{R}}{8} + {\frac{P_{B}}{8} \pm {\frac{\sqrt{P_{R}P_{B}}}{4}{{\sin \left\lbrack {{\left( {\omega_{R} - \omega_{B}} \right)\quad t} + \quad \left( {\varphi_{R} - \varphi_{B}} \right) + \left( {{2\theta_{BC}} + \theta_{BF} - \theta_{{AH} + {H\quad F}}} \right) + {2\varphi_{{CL}_{R}}}} \right\rbrack}.}}}}} & (8)\end{matrix}$

[0052] The upper (plus) sign in the equation corresponds to the opticalpower in the upper waveguide exiting the coupler F while the lower(minus) sign corresponds to the optical power in the lower waveguideexiting the coupler F. The phase angle 2θ_(BC)+θ_(BF)−θ_(AH+HF)≡θ_(F) ismade small and temperature invariant or insensitive by setting thepathlengths 2BC+BF=AH+HF. Finally, the optical intensity interferencepattern at the optical signal output coupler, I, is given by thefollowing formula 9 $\begin{matrix}{\left. {P_{I \pm} = {\frac{P_{R}}{8} + {\frac{P_{B}}{8} \pm {\frac{\sqrt{P_{R}P_{B}}}{4}{\sin\left\lbrack {{\left( {\omega_{R} - \omega_{B}} \right)\quad t} + \quad \left( {\varphi_{R} - \varphi_{B}} \right) + \theta_{{BG} + {GI}} - \theta_{{AH} + {H\quad I}}} \right)}}}}} \right\rbrack.} & (9)\end{matrix}$

[0053] The upper (plus) sign in the equation corresponds to the opticalpower in the upper waveguide exiting the coupler I while the lower(minus) sign corresponds to the optical power in the lower waveguideexiting the coupler I. The phase angle θ_(BG+GI)−θ_(AH+HI)≡θ_(I) is madesmall and temperature invariant or insensitive by setting thepathlengths BG+GI=AH+HI. From the phase of this last equation, theunknown and uncontrolled phase term, φ_(R)−φ_(B), may be obtained for anindependent determination of L_(R) and L_(B) in the two precedingequations.

[0054]FIG. 6 illustrates an embodiment in which the additional couplersG and H are arranged such that the input phase signal is coupled out onthe right side of the PLC. This embodiment is referred to herein as the“Jellyfish”. The design and operation of this circuit is essentially thesame as the Lightbulb. If the optical signal inputs are as described forthe Trombone, P_(R)(ω_(R), φ_(R)) into coupler B and P_(B)(ω_(B), φ_(B))into coupler A, then the optical intensity interference pattern at theoptical signal output coupler, E, is given by the following formula 9$\begin{matrix}{\left. {\left. {P_{E \pm} = {\frac{P_{R}}{8} + {\frac{P_{B}}{8} \pm {\frac{\sqrt{P_{R}P_{B}}}{4}{\sin\left\lbrack {{\left( {\omega_{R} - \omega_{B}} \right)\quad t} + \quad \left( {\varphi_{R} - \varphi_{B}} \right) + \theta_{{BH} + {HE}} - {2\theta_{AD}} - \theta_{AE}} \right)}}}}} \right\rbrack - {2\varphi_{{DL}_{R}}}} \right\rbrack.} & (9)\end{matrix}$

[0055] The upper (plus) sign in the equation corresponds to the opticalpower in the upper waveguide exiting the coupler E while the lower(minus) sign corresponds to the optical power in the lower waveguideexiting the coupler E. The phase angle θ_(BH+HE)−2θ_(AD)−θ_(AE)≡θ_(E) ismade small and temperature invariant or insensitive by setting thepathlengths BH+HE=2AD+AE.

[0056] The optical intensity interference pattern at the optical signaloutput coupler, F, is given by the following formula 10 $\begin{matrix}{P_{F \pm} = {\frac{P_{R}}{8} + {\frac{P_{B}}{8} \pm {\frac{\sqrt{P_{R}P_{B}}}{4}{{\sin \left\lbrack {{\left( {\omega_{R} - \omega_{B}} \right)\quad t} + \quad \left( {\varphi_{R} - \varphi_{B}} \right) + \left( {{2\theta_{BC}} + \theta_{BF} - \theta_{{AG} + {GF}}} \right) + {2\varphi_{{CL}_{R}}}} \right\rbrack}.}}}}} & (10)\end{matrix}$

[0057] The upper (plus) sign in the equation corresponds to the opticalpower in the upper waveguide exiting the coupler F while the lower(minus) sign corresponds to the optical power in the lower waveguideexiting the coupler F. The phase angle 2θ_(BC)+θ_(BF)−θ_(AG+GF)≡θ_(F) ismade small and temperature invariant or insensitive by setting thepathlengths 2BC+BF=AG+GF.

[0058] Finally, the optical intensity interference pattern at theoptical signal output coupler, I, is given by the following formula 11$\begin{matrix}{P_{I \pm} = {\frac{P_{R}}{8} + {\frac{P_{B}}{8} \pm {\frac{\sqrt{P_{R}P_{B}}}{4}{\sin\left\lbrack {{\left( {\omega_{R} - \omega_{B}} \right)\quad t} + \quad \left( {\varphi_{R} - \varphi_{B}} \right) + \theta_{{BH} + {HI}} - \theta_{{AG} + {G\quad I}}} \right\rbrack}}}}} & (11)\end{matrix}$

[0059] The upper (plus) sign in the equation corresponds to the opticalpower in the upper waveguide exiting the coupler I while the lower(minus) sign corresponds to the optical power in the lower waveguideexiting the coupler I. The phase angle θ_(BH+HI)−θ_(AG+GI)≡θ_(I) is madesmall and temperature invariant or insensitive by setting thepathlengths BH+HI=AG+GI. From the phase of this last equation, theunknown and uncontrolled phase term, φ_(R)−φ_(B), may be obtained for anindependent determination of LR and LB in the two preceding equations.

[0060] Aspects of the present invention provide unique properties invarious applications. For example, the present invention can provide anumber of unique properties in the fabrication process. Along theselines, the present invention may be employed to replace a number ofdiscreet, precision optical components and the attendant assemblyprocess with a single component manufactured with standard industrialprocesses which have grown out of a combination of optical fiberfabrication technology and microelectronic large scale integrationtechnology. FIG. 7 illustrates a perspective view of an embodiment ofthe PLC described in conjunction with FIGS. 3 and 4. If a planarlightwave circuit according to the present invention employs telecomindustry wavelengths, there is no change in nor perturbation to standardPLC process lines. Only the photolithographic mask to define thespecific circuit is required as a special item.

[0061] The optical waveguide size and minimum bending radii may bedetermined by the specific index contrast utilized in a particularfabrication facility or process. Representative values are given inTable 1 from M. Kawachi, Recent progress in silica-based planarlightwave circuits on silicon, IEE Proc.-Optoelectron., Vol. 143, pp.257-262 (1996). Table 1 Silica on Silicon Waveguide Parameters Waveguidetype Low Δ Middle Δ High Δ Core/cladding 0.25 0.45 0.75 index contrastΔ(%) Core size, μm 8 × 8 7 × 7 6 × 6 Loss, dB/cm <0.1 <0.1 <0.1 Fibercoupling <0.1 0.1 0.5 loss, dB/point Minimum bending 25 15 5 radius, mm

[0062] From the PLC circuits laid out as schematically indicated inFIGS. 2 and 4-6, the minimum bending radii given in Table 1 willestablish typical minimum PLC circuit dimensions of greater that about 5cm by about 5 cm for a low contrast process to about 1 cm by about 1 cmfor a high contrast process since all circuits involve arcs of about 90°or about 180° in various orientations.

[0063] The total thickness of glass on the silicon substrate typicallyvaries from about 10 to about 50 μm. This thickness is dependent on theestablished processes of various manufacturers and does not affect theoperation of the PLC. It may, however, influence the post processfabrication of TM mode stripping structures as discussed below.

[0064] In the analysis, it was assumed that critical phase relationswere established at the input couplers (A and B in FIGS. 1-6) andpersisted only through the output couplers (E, F and I in the Figures).This is true since the phase of the two signals exiting coupler A arefixed with respect to one another at the input of coupler A as are thetwo signals exiting coupler B fixed with respect to one another at theinput of coupler B. When the signals exit the couplers E, F or I, thephase of the heterodyne or beat note between the two signals isestablished and no further interaction occurs. Thus, the entireheterodyne interaction occurs in the robust, monolithic,photolithographically defined structure of silica on silicon.

[0065] Other advantages of the present invention are achieved throughthe balanced detection possible. The discussion above assumes that allcouplers, combiners or splitters were 50::50. The output couplers shouldall be 50::50 for balanced detection optimization. However, there may beadvantages to using to using other splitting ratios in certain cases.For example, FIG. 8 schematically represents the output of a balanceddetector as being the difference in photocurrents generated in twoseries connected photodiodes. The input optical powers are P_(R)(ω_(R),θ_(R)) to the upper waveguide and P_(B)(ω_(B), θ_(B)) to the lowerwaveguide. The coupling of energy from the lower to upper waveguide andvisa versa is dependent upon the coupling constant between thewaveguides and the length of the interacting region. There is asinusoidal variation of the coupling with length and coupling constantas indicated in FIG. 8. If the coupling constant and interaction lengthare such that kL=π/4, then the coupling ratio is 50::50 and thephotocurrent, i_(sig), in the balanced detector contains only the beatfrequency or heterodyne term. Also, with the photoresponce of thephotodetectors, A_(res) amps/watt, identical for both the upper andlower photodiodes in FIG. 8, the output signal current is maximum forkL=π/4, i_(sig)=−2A_(res) {square root}{square root over (P_(R)P_(B))}sin[(ω _(R)−ω_(B))t+θ_(R)−θ_(B)] Further more, if the total opticalpower into the coupler, P_(opt)=P_(R)+P_(I) is fixed, then the signalcurrent is maximized for P_(R)=P_(B)=P_(opt)/2.

[0066] If the total input optical power to the PLC is fixed by thecommon source laser indicated in FIG. 1 or 3 but the optical loss in onechannel is excessive (typically in the measurement paths to the externalmirrors), then further circuit optimization may be possible by adjustingthe coupling ratios of the input couplers, A and B. In the case of theracetrack circuit, FIGS. 1 and 2, the input coupler splitting ratio maybe designed to provide excess power into the measurement circuit. In thecase of the lightbulb and jellyfish circuits shown in FIGS. 5 and 6, thereference splitters may be designed to optimize the division of powerbetween the reference output port at I and the measurement ports at Eand F.

[0067] Further advantages of the present invention relate to temperatureinsensitivity of the devices. In the description of the variousembodiments of the PLC circuit, certain path lengths were set equal tominimize thermal effects. These are summarized in Table 2. In everycase, the small phase off-set terms may be written as(ω_(R)n_(eff)/c)[ΔL−LΔω)/ω_(R)] or (2πn_(eff)/λ_(oR))[ΔL−LΔω)/ω_(R)]where λ_(oR) is the free space wavelength of the optical signal atfrequency ω_(R). Table 2 Phase imbalance or off-set terms CircuitMatching Paths Phase Off-set Racetrack AE = BC + DE θ_(E) =ω_(R)n_(eff)[BC + DE − Figures 1-2 AE(1 + ″ω/ω_(R))]/c AI = BI θ_(E) =ω_(R)n_(eff)[BI − AI(1 + Δω/ω_(R))]/c Trombone BE = 2AD + AE θ_(E) =ω_(R)n_(eff)[BE − (BE − Figures 3-4 (2AD + AE)(1 + Δω/ω_(R))]/c AF =2BC + BF θ_(E) = ω_(R)n_(eff)[2BC + BF − AF(1 + Δω/ω_(R))]/c LightbulbBG + GE = 2AD + AE θ_(E) = ω_(R)n_(eff)[BG + GE − (2AD + AE)(1 +Δω/ω_(R))]/c AH + HF = 2BC + BF θ_(E) = ω_(R)n_(eff)[2BC + BF − (AH +HF)(1 + Δω/ω_(R))]/c AH + HI = BG + GI θ_(E) = ω_(R)n_(eff)[BG + GI −(AH + HI)(1 + Δω/ω_(R))]/c Jellyfish BH + HE = 2AD + AE θ_(E) =ω_(R)n_(eff)[BH + HE − (2AD + AE)(1 + Δω/ω_(R))]/c AG + GF = 2BC + BFθ_(E) = ω_(R)n_(eff)[2BC + BF − (AG + GF)(1 + Δω/ω_(R))]/c AG + GI =BH + HI θ_(E) = ω_(R)n_(eff)[BH + HI − (AG + GI)(1 + Δω/ω_(R))]/c

[0068] The length ΔL for example BC+DE-AE in the first line of Table 2may easily be maintained at less than about 5 micrometers by thephotolithographic design/fabrication process, including the location ofthe PLC edges at C and D in FIGS. 1-6. The edges of the PLC may beidentified by a series of fiducial lines included on thephotolithographic mask and precision edge polishing following PLCfabrication. The properties of silica on silicon PLCs is such that thethermal coefficient of optical path length changes in a silica-basedwaveguide is${\frac{1}{\Delta \quad L}\frac{\quad}{T}\left( {n_{eff}\Delta \quad L} \right)} = {1 \times {10^{- 5}\left\lbrack {1/{{\,^{\circ \quad}C}.}} \right\rbrack}}$

[0069] so that if ΔL ≦about 5 μm, δΔL ≦about 5×10⁻⁵δT μm. A change intemperature of approximately one degree Celsius will result in onlyabout 50 picometers of PLC unbalance change.

[0070] From the embodiments of heterodyne interferometer PLCs indicatedin FIGS. 2 and 4-6, the PLC optical path lengths of all paths from A orB to E, F, or I will be L˜2πR_(min), where R_(min) is the minimum radiusof curvature from Table 1. Since 5≦R_(min)≦25 mm and10⁻¹⁰≦Δω/ω_(R)≦10⁻⁷, the maximum thermal contribution of then_(eff)LΔω/ω_(R) term to the optical pathlength change will beδL≦1.6×10⁻⁸δT micrometers. A temperature change of about one degreeCelsius will result in only about 0.016 picometers of PLC unbalancechange, which is completely negligible compared to the thermal effectsassociated with ΔL changes.

[0071] Still further advantages of the present invention relate toimmunity of devices according to the present invention from spurioussignals. Spurious signals at the output couplers at the two frequenciesω_(R) and ω_(B), which have traversed paths other than the intended pathwill introduce measurement errors. These spurious signals may arise fromreflections (especially at PLC interfaces with the external world),cross coupling (at waveguide crossings) and polarization modedispersion.

[0072] The use of “angle lapping” is well known in the fiber opticindustry to reduce reflections at fiber-fiber interfaces. This techniqueis also used for fiber-PLC interfaces. An additional interface refectionreduction at fiber-PLC interfaces is accomplished by the use of indexmatching bonding agents at the silicon V-block fiber assembly structuresindicated in FIGS. 1 and 3. For the heterodyne interferometer,reflection reduction at the PLC-free space interfaces, ports C and D inthe embodiments shown in FIGS. 1-7, by angle lapping will result inreflection reduction as indicated in FIG. 9. Further reduction may beachieved by anti-reflection coating of the PLC-free space interface.

[0073] Single mode optical fibers and single mode waveguides in PLCs maysustain two orthogonal polarizations that may propagate at slightlydifferent velocities. It may therefore be necessary to preferentiallyexcite only one polarization mode at each input port, such as ports Aand B in the embodiments shown in FIGS. 1-7, and to remove any opticalpower scattered from the desired mode into the orthogonal mode in theinterferometer. This “mode stripping” operation may be accomplished atthe output ports, such as ports E, F and I in the embodiments shown inFIGS. 1-7.

[0074] Preferential excitation of the desired mode may be accomplishedby transporting the polarized source light to the PLC by polarizationmaintaining fibers as indicated in FIGS. 1 and 3. A mode strippingstructure may be fabricated in PLCs as indicated in FIG. 11. Thisstructure can operate on the differing optical currents associated withthe TE and TM modes that are induced in a metallic conductor. The TMoptical mode is characterized by a large transverse optical frequencymagnetic field oriented parallel to the plane of the PLC while the TEoptical mode is characterized by a large optical frequency magneticfiled oriented perpendicular to the plane of the PLC. The tangentialmagnetic field induces optical currents in adjacent metallic conductors.FIG. 11 illustrates optical attenuation in the TM and TE modes of a highAn single mode waveguide due to an AI metal film. The thick topwaveguide cladding layer may be thinned to the desired depth by variousprocesses such as reactive ion etching, ion beam milling and/or wetchemical etching. The chosen etch procedure may be adjusted to provide asmooth surface for the depostion of the metallic film. The chosenexample metal, Al, was selected due to the large magnitude of both thereal, n, and imaginary, k, components of the optical index for AI at awavelength of about 1.56 μm. These mode stripping patches may be locatedoutside of the critical PLC paths before the input couplers A and B andafter the output couplers E, F, and I in FIGS. 1-7.

[0075] The foregoing description of the invention illustrates anddescribes the present invention. Additionally, the disclosure shows anddescribes only the preferred embodiments of the invention, but asaforementioned, it is to be understood that the invention is capable ofuse in various other combinations, modifications, and environments andis capable of changes or modifications within the scope of the inventiveconcept as expressed herein, commensurate with the above teachings,and/or the skill or knowledge of the relevant art. The embodimentsdescribed hereinabove are further intended to explain best modes knownof practicing the invention and to enable others skilled in the art toutilize the invention in such, or other, embodiments and with thevarious modifications required by the particular applications or uses ofthe invention. Accordingly, the description is not intended to limit theinvention to the form disclosed herein. Also, it is intended that theappended claims be construed to include alternative embodiments.

We claim:
 1. A method for performing optical signal and beamdistribution in a heterodyne interferometer, the method comprising:providing a planar lightwave circuit comprising a plurality of waveguideoptical transmission elements and an input coupler and an output couplerarranged along the optical transmission elements; matching opticalpathlengths of the transmission elements between the input coupler andthe output coupler to compensate for thermal effects; and determiningreference and measurement optical phases employing the input coupler andthe output coupler.
 2. The method according to claim 1, wherein theinput coupler and the output coupler comprise optical waveguidedirectional couplers.
 3. The method according to claim 1, wherein theinput coupler and the output coupler comprise multimode interference(MMI) devices.
 4. The method according to claim 1, wherein the inputcouplers comprise waveguide Y-branch couplers.
 5. The method accordingto claim 1, wherein the output coupler comprises a waveguide directionalcoupler with a 50:50 splitting ratio.
 6. The method according to claim1, wherein the output directional couplers are operable to provide adifferential output appropriate for balanced detection.
 7. The methodaccording to claim 1, wherein the output couplers comprise a 2×2multimode interference device operable to provide a differential outputappropriate for balanced detection.
 8. The method according to claim 1,wherein the output coupler comprises a 2×1 combiner operable to providea single ended output.
 9. The method according to claim 1, furthercomprising: utilizing at least one of the input coupler and the outputcoupler to split off a reference phase signal; and selecting a couplingratio for at least one of the input coupler and the output coupler tooptimize a detected heterodyne output signal when unequal losses areencountered in either measurement optical paths or reference opticalpaths.
 10. The method according to claim 1, further comprising:fabricating the planar lightwave circuit in silica on silicon.
 11. Themethod according to claim 10, further comprising: fabricating the planarlightwave circuit in silica on silicon utilizing planar lightwavefabrication processes.
 12. The method according to claim 1, furthercomprising: fabricating the planar lightwave circuit in silica onquartz.
 13. The method according to claim 1, further comprising:fabricating the planar lightwave circuit from at least one of a polymer,a III-V semiconductor, silicon, and lithium niobate.
 14. The methodaccording to claim 1, further comprising: achieving dimensional controlof waveguide and device critical dimensions of the planar lightwavecircuit utilizing microelectronic photolithographic techniques toprovide the planar lightwave circuit.
 15. The method according to claim1, further comprising: achieving dimensional control of matched planarlightwave circuit waveguide lengths utilizing microelectronicphotolithographic techniques.
 16. The method according to claim 1,further comprising: designing crossings of the transmission elements forapplication specific require, minimal crosstalk.
 17. The methodaccording to claim 1, further comprising: fabricating selected modepolarization strippers at an input port and an output port of the planarlightwave circuit.
 18. The method according to claim 17, furthercomprising: positioning a metal layer above or below the planarlightwave circuit; and inducing optical evanescent H-field currents inthe metal to selectively strip a TM polarization mode off at the inputand output ports.
 19. A device operable to distribute optical signalsand beams in a heterodyne interferometer, the device comprising: aplanar lightwave circuit comprising a plurality of waveguide opticaltransmission elements; and an input coupler and an output couplerarranged along the optical transmission elements and operable todetermine reference and measurement optical phases, wherein opticalpathlengths of the optical transmission elements between the inputcoupler and the output coupler are matched to compensate for thermaleffects.
 20. The device according to claim 19, wherein the couplerscomprise optical waveguide directional couplers.
 21. The deviceaccording to claim 19, wherein the couplers comprise multimodeinterference devices.
 22. The device according to claim 19, wherein thecouplers comprise waveguide Y-branch couplers.
 23. The device accordingto claim 19, wherein the output coupler comprises a waveguidedirectional coupler having a 50:50 splitting ratio.
 24. The deviceaccording to claim 23, wherein the output coupler is operable to providea differential output appropriate for balanced detection.
 25. The deviceaccording to claim 20, wherein the output coupler is operable to providea differential output appropriate for balanced detection.
 26. The deviceaccording to claim 19, wherein the output coupler comprises a 2×2multi-mode interference device operable to provide a differential outputfor balanced detection.
 27. The device according to claim 19, whereinthe output coupler comprises a 2×1 combiner operable to provide a singleended output.
 29. The device according to claim 19, wherein at least oneof the input coupler and the output coupler is operable to split off areference phase signal.
 30. The device according to claim 19, wherein atleast one of the input coupler has a coupling ratio operable to optimizea detected heterodyne output signal when encountering unequal losses inmeasuring optical paths or reference optical paths.
 31. The deviceaccording to claim 19, wherein the optical transmission elements areembedded in a silica layer.
 32. The device according to claim 19,wherein the substrate is silicon.
 33. The device according to claim 19,wherein the substrate is quartz.
 34. The device according to claim 19,wherein the planar lightwave circuit comprises at least one of apolymer, a III-V semiconductor, silicon and lithium niobate.
 35. Thedevice according to claim 19, wherein the planar lightwave circuitfurther comprises: crossings of the waveguide optical transmissionelements, the waveguide crossings being operable for applicationspecific required minimal crosstalk.
 36. The device according to claim19, further comprising: selected mode polarization strippers arranged atan input port and an output port of the planar lightwave circuit. 37.The device according to claim 36, wherein the TM polarization mode isselectively stripped off at the input and output ports by the use ofoptical evanescent H-field induced currents in an appropriatelypositioned metal above or below the optical waveguide.